Counting semi-viable bacteria in cheese

The Wageningen researcher Christine Bunthof has developed a direct method for counting bacteria in dairy products. The method not only distinguishes viable and non-viable bacteria but also semi-viable bacteria. These are too weak to divide, but still exhibit activity. The semi-viable bacteria play an important role in cheese ripening and therefore influence the taste.

With the new counting method, developers of probiotic dairy products can also investigate how active the added bacteria are when they enter the gastrointestinal tract.

The microbiologist from Wageningen University stained the bacteria with two fluorescent substances. One substance ensures that the bacteria emit a green colour if they are active. The substance itself is not fluorescent but is converted by bacterial enzymes into a green fluorescent dye. The second substance stains bacteria with a damaged membrane. This substance emits red but only when it is bound to the bacteria`s DNA.

The method distinguishes viable from non-viable types of bacteria. It also detects semi-viable bacteria as, although these can no longer divide, they are still active and therefore emit green. Only non-viable bacteria emit red. Christine Bunthof used a flow cytometer to count the bacteria. In this device, the bacteria are transported one by one past a laser which lights up the bacteria. A detector counts the number of red and green bacteria.

Since time immemorial the diary industry has always counted bacteria which can still divide. When cultured on a nutrient medium, these bacteria form visible colonies. However, this old method fails to detect semi-viable bacteria, as active bacteria which do not multiply remain invisible. Yet this intermediate group constitute a considerable proportion of the active bacteria in ripening cheese.

This counting method is not only useful for following the ripening of cheese and yoghurt. It is also important for the development of probiotic dairy products. These products contain bacteria with a healthy effect. However, some bacteria do not survive the acidic gastric juices and the bile in the small intestine. Using this new method, producers can test how well bacteria can reach the intestines in an active form.